Recently, the performance and functionality of digital cameras and digital movie cameras that use some solid-state imaging device such as a CCD and a CMOS (which will be sometimes referred to herein as an “image sensor”) have been enhanced to an astonishing degree. In particular, the size of a pixel structure for use in a solid-state imaging device has been further reduced these days thanks to development of semiconductor device processing technologies, thus getting an even greater number of pixels and drivers integrated together in a solid-state imaging device. As a result, the resolution of an image sensor has lately increased significantly from one million pixels to ten million pixels in a matter of few years. Meanwhile, the greater the number of pixels in an image sensor, the lower the intensity of the light falling on a single pixel (which will be referred to herein as a “light intensity”) and the lower the sensitivity of the image capture device tends to be.
On top of that, in a normal color camera, a subtractive organic dye filter (color filter) that uses an organic pigment as a dye is arranged over each photosensing section of an image sensor, and therefore, the optical efficiency achieved is rather low. In a Bayer color filter, which uses a combination of one red (R) pixel, two green (G) pixels and one blue (B) pixel as a fundamental unit, the R filter transmits an R ray but absorbs G and B rays, the G filter transmits a G ray but absorbs R and B rays, and the B filter transmits a B ray but absorbs R and G rays. That is to say, each color filter transmits only one of the three colors of R, G and B and absorbs the other two colors. Consequently, the light ray used by each color filter is only approximately one third of the visible radiation falling on that color filter.
To overcome such a problem of decreased sensitivity, Patent Document No. 1 discloses a technique for increasing the intensity of the light received by attaching an array of micro lenses to a photodetector section of an image sensor. According to this technique, the incoming light is condensed with those micro lenses, thereby substantially increasing the aperture ratio. And this technique is now used in almost all solid-state imaging devices. It is true that the aperture ratio can be increased substantially by this technique but the decrease in optical efficiency by color filters still persists.
Thus, to avoid the decrease in optical efficiency and the decrease in sensitivity at the same time, Patent Document No. 2 discloses a solid-state imaging device that has a structure for taking in as much incoming light as possible by using dichroic mirrors and micro lenses in combination. Such a device uses a combination of dichroic mirrors, each of which does not absorb light but selectively transmits only a component of light falling within a particular wavelength range and reflects the rest of the light falling within the other wavelength ranges. Each dichroic mirror selects only a required component of the light, directs it toward its associated photosensing section and transmits the rest of the light. FIG. 9 is a cross-sectional view of such an image sensor as the one disclosed in Patent Document No. 2.
In the image sensor shown in FIG. 9, the light that has reached a condensing micro lens 11 has its luminous flux adjusted by an inner lens 12, and then impinges on a first dichroic mirror 13, which transmits a red (R) ray but reflects rays of the other colors. The light ray that has been transmitted through the first dichroic mirror 13 is then incident on a photosensitive cell 23 that is located right under the first dichroic mirror 13. On the other hand, the light ray that has been reflected from the first dichroic mirror 13 impinges on a second dichroic mirror 14 adjacent to the first dichroic mirror 13. The second dichroic mirror 14 reflects a green (G) ray and transmits a blue (B) ray. The green ray that has been reflected from the second dichroic mirror 14 is incident on a photosensitive cell 24 that is located right under the second dichroic mirror 14. On the other hand, the blue ray that has been transmitted through the second dichroic mirror 14 is reflected from a third dichroic mirror 15 and then incident on a photosensitive cell 25 that is located right under the dichroic mirror 15. In this manner, in the image sensor shown in FIG. 9, the visible radiation that has reached the condensing micro lens 11 is not lost but their RGB components can be detected by the three photosensitive cells non-wastefully.
Meanwhile, Patent Document No. 3 discloses a technique that uses a micro prism. According to that technique, the incoming light is split by the micro prism 16 into red (R), green (G) and blue (B) rays, which are received by their associated photosensing sections as shown in FIG. 10. Even with such a technique, the R, G and B components can also be detected with no optical loss caused.
According to the techniques disclosed in Patent Documents Nos. 2 and 3, however, the number of photosensing sections to provide needs to be as many as that of the color components to separate. That is why to receive red, green and blue rays that have been split, for example, the number of photosensing sections provided should be tripled compared to a situation where conventional color filters are used.
Thus, to overcome such problems with the prior art, Patent Document No. 5 discloses a technique for increasing the optical efficiency by using dichroic mirrors and reflected light, although some loss of the incoming light is involved. FIG. 11 is a partial cross-sectional view of an image sensor that adopts such a technique. As shown in FIG. 11, dichroic mirrors 32 and 33 are embedded in a light-transmitting resin 31. Specifically, the dichroic mirror 32 transmits a G ray and reflects R and B rays, while the dichroic mirror 33 transmits an R ray and reflects G and B rays.
Such a structure cannot receive a B ray at its photosensing section but can sense R and G rays with no loss under the following principle. First, if an R ray impinges on the dichroic mirrors 32 and 33, the R ray is reflected from the dichroic mirror 32, is totally reflected from the interface between the light-transmitting resin 31 and the air, and then impinges on the dichroic mirror 33. Then, almost all of the R ray that has impinged on the dichroic mirror 33 is transmitted through the organic dye filter 35 and the micro lens 36 that transmit the R ray and then incident on the photosensing section, even though only a part of the R ray is reflected from the metal layer 37. On the other hand, if a G ray impinges on the dichroic mirrors 32 and 33, the G ray is reflected from the dichroic mirror 33, is totally reflected from the interface between the light-transmitting resin 31 and the air, and then impinges on the dichroic mirror 32. Then, almost all of the G ray that has impinged on the dichroic mirror 32 is transmitted through the organic dye filter 34 and the micro lens 36 that transmit the G ray and eventually incident on the photosensing section with virtually no loss.
According to the technique disclosed in Patent Document No. 5, only one of the three color components of RGB is lost but light rays of the other two colors can be received with almost no loss based on the principle described above. That is why there is no need to provide photosensing sections for all of the three colors of RGB. In this case, comparing such an image sensor to an image sensor that uses only organic dye filters, it can be seen that the optical efficiency can be doubled by this technique. Still, even if such a technique is adopted, the optical efficiency cannot be 100%, as one out of the three colors should be sacrificed.
Meanwhile, to increase the horizontal and vertical resolutions of an image capture device, a so-called “pixel shifted” arrangement, in which the pixels are shifted by a half pitch between two adjacent rows and between two adjacent columns, is sometimes adopted instead of the conventional square arrangement. An image sensor in which pixels are shifted both horizontally and vertically is disclosed in Patent Document No. 5, for example. In such an image sensor, a photosensing section is arranged in a diamond shape so that signals supplied from respective pixels are read out from CCDs that are arranged in a zigzag pattern. By shifting the pixels by a half pitch both horizontally and vertically, the horizontal and vertical resolution can be both increased.
Citation List
Patent Literature
                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 59-90467        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2000-151933        Patent Document No. 3: PCT International Application Japanese National Phase Publication No. 2002-502120        Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 2003-78917        Patent Document No. 5: Japanese Patent Application Laid-Open Publication No. 60-187187        